1. Field of the Invention
This invention relates to synthetic yarn coated with ionomer resin, methods of coating the yarn with ionomer resin, and methods of forming fabric from the ionomer-coated yarn.
More particularly, the invention is directed to wet press felts for use in papermaking machines wherein the wet press felts comprise a base layer which is a fabric made from ionomer-coated yarn in combination with a fibrous batt, the fibrous batt and ionomer-coated base layer joined by a needling process.
2. Description of the Prior Art
In general, papermakers' wet felts comprise an endless belt having a smooth paper-receiving surface and an abrasion-resistant machine roll contacting surface. The press felts perform the function of supporting the paper web during the dewatering process, advancing the paper web to the next stage in the papermaking process, and serving as a receptacle for water removed during the dewatering process, the water also being conveyed away from the processing by the press felt.
During processing as it is known in today's papermaking industry, tremendous roll pressures and high speeds are developed. Accordingly there is a tendency for the wet press felts to wear rapidly due to abrasion and, additionally, to lose porosity and therefore the ability to remove water from the paper web.
Since the 1960's, the paper industry has witnessed and experienced the impact of a paper machine clothing evolution, especially in the press section. The older batt-on-base needled felts have been replaced by more sophisticated batt-on-mesh needled designs which yield greater dewatering efficiencies and longer felt life. These early needled felts had a base woven from spun staple fibers into which the batt fibers, generally nylon, were inserted by the needling process. The weight percentage of the base fibers was relatively high and these fibers were mainly oriented in the machine and cross-machine directions. Thus only a small amount of the capillary volume in the felt was formed by the batt fibers which were mainly vertically oriented. When these felts underwent dewatering on the paper machine by passing through the nip of squeeze rollers, the fibers in the base fabric became compressed, particularly at the crossover points. As the base fibers compressed, the overall felt rapidly lost porosity and capillary structure, yielding poor dewatering properties. Additionally, the compressed fibers increasingly trapped fine particles from both paper and fractured felt fibers which further led to early felt failure.
The newer batt-on-mesh felts utilize high-strength twisted multifilament yarns or monofilaments. The mesh base not only has greater open area between machine and cross-machine yarns, but also resists compaction to a greater degree than the spun yarn base. This greater initial porosity and resistance to compaction has resulted in improved felt life. However, these yarns have the disadvantage of reduced retention of the batt fibers and lower abrasion resistance of the mesh under certain conditions due to fibrillation of the filament surface.
Also known to the prior art is a mesh base which contains highly twisted multifilaments, usually nylon, impregnated with phenolic resin. These twisted yarns are impregnated before weaving by immersing the yarns in a solution of phenolic resin followed by drying at a sufficiently low temperature to evaporate the solvent without curing the resin. The treated yarns in this state remain reasonably flexible to facilitate weaving of the mesh fabric. After needling the batt onto the mesh, the felt is then heat treated at a temperature which will crosslink the phenolic resin into a finally cured stage. However, these phenolic resin-impregnated yarns suffer from poor batt retention and are difficult to weave due to the high friction surface characteristics of the impregnated material.
Fleischer, U.S. Pat. No. 4,093,512, discloses a papermakers' belt comprising ultrahigh modulus load bearing yarns which may be resin coated or wrapped and then resin coated to improve their abrasion resistance. Among the resins suggested for coating the high modulus yarn are acrylic resins, phenolic resins, and nylon resins. The reference fails to disclose any suggestion of the use of an ionomer resin as a coating for synthetic yarns.
Khan, U.S. Pat. No. 4,259,394, discloses a papermaking fabric composed of a base having a fibrous batt needled to one surface thereof. The base is formed of interwoven core wrapped yarns comprising core yarns which are effectively heat infusible and wrapping yarns which are effectively heat fusible. Polyethylene and polypropylene are the only resins disclosed as wrapping materials.
Thus a need has continued to exist for a yarn suitable for forming the mesh layer of papermakers' wet felt which has good abrasion resistance, good durability, good toughness, good resistance to compression, good adhesion at the crossover points, low frictional characteristics, good needling characteristics regarding both integrity of the base yarns and batt retention, and good handling characteristics during weaving.